I'm not encouraging you to modify LEC facilties, but it's probably HDSL2 on the other side of the NIU. Take it off the pole, move it to the trailer, and use your other cable to extend the HDSL2 side, not the DSX-1 side.
Not that I've ever done that, but in case I get the urge I have a 4 slot mounting with the variations in HDSL4, HDSL2, and ADC HLXR cards "just in case".